1. Field of the Invention
The invention relates generally to antennas for communication and more specifically to a near-field magnetic communication antenna.
2. Description of the Prior Art
Magnetic near-field communication provides secure communications with uses that include but are not limited to access control, financial transactions, and security systems. Some systems for near-field communication use a transmit antenna and a separate receive antenna. Coupling of the transmit magnetic field to the receive antenna plagues these types of systems because the transmit signal voltage is orders of magnitude greater than the desired receive signal voltage. For example, in some systems, the transmit signal voltage is about 400 volts and the desired receive signal voltage is about one volt or less. Therefore, to avoid overloading the receiver circuitry, transmit/receive switches are currently used to isolate the receiver from the transmitter when the transmitter is transmitting. These switches add significant cost and complexity to near-field communication systems due to the cost of the switch itself and also due to the need to control the transmit/receive switch to ensure the receiver is protected. The use of a transmit/receive switch, however, does not allow simultaneous transmitting and receiving of signals. Therefore, such a system is restricted to half-duplex communication only.
FIG. 1 shows a block diagram of typical transceiver 4 of the prior art. The transceiver 4 includes a transmitter 1, a receiver 2, and a transmit/receive switch 3. The transmitter 1 is connected to a transmit coil 5 and the receiver 2 is connected to a receive coil 6 via the transmit/receive switch 3. When the transmitter 1 is transmitting, the transmit/receive switch 3, is switched to ground 7 so that no current or voltage from the transmitted signal is coupled into the receiver 2. As a result, the receiver 2 is protected from the high-power transmit signal. When the transceiver 4 is in receive mode, the transmit/receive switch 3 connects the receiver 2 to the receive coil 6, thereby allowing the receiver 2 to receive the desired signal. When the transceiver 4 is receiving, the transmitter 2 is turned off.
In some embodiments of the prior art, the transmit coil and the receive coil are the same coil. In such a system with only one coil, the transmit/receive switch 3 switches between the transmitter 1 and the receiver 2 depending on the state of the transceiver 4. Therefore, simultaneous transmitting and receiving is not possible.
Much work has been done to optimize loop antennas for electronic article surveillance systems. The prior art includes antennas disclosed by Lichtblau (U.S. Pat. Nos. 3,938,044 and 4,251,808), Bowers et al. (U.S. Pat. Nos. 5,602,556 and 5,914,692), Stewart (U.S. Pat. No. 8,854,188), Rhodes et al. (GB 2,475,842A and US 20090160725A1), and Manov et al. (U.S. Pat. No. 6,836,216). All of these patent references disclose antenna designs that use a single function transmit antenna and a single function receive antenna that are separated by a small distance. These systems detect signals within a range of a few meters and are not used for communication.
Rhodes et al. disclose a transmit-receive antenna that has a transmit loop antenna and a receiver solenoid antenna positioned with its axis in the plane of the transmit loop antenna. The collocated transmit and receive antennas reduce the signal generated in the solenoid receiver antenna during transmit because the receive coils are aligned orthogonally to the magnetic field generated by the transmit coil.
Manov et al. disclose an electronic article surveillance system that has two antenna arrays placed in a spaced-apart parallel relationship, where each antenna consists of two coil assemblies. Each coil assembly has a transmitting coil, a receiving coil, and a compensating coil, where windings of the receiving coil are farther from the windings of the transmitting coil than those of the compensating coil. Antennas are powered in different phases to generate a magnetic field of different orientations within an interrogation zone between the arrays. When a marker is placed near the antenna assembly, the field disturbances produced by its re-magnetization induce voltages in both the receiving coil and in the compensating coils. When used in a theft-prevention system, for example, a person passing through the gap (i.e., the interrogation zone) between the transmit and receive antennas while carrying an article containing a surveillance marker would be detected based on disturbances in the magnetic field. To prevent false alarms due to a shopper merely passing near the system with a surveillance marker, these systems use signal phasing in multiple loops to reduce the range of the signal.
Stewart discloses a radio frequency identification (RFID) system that includes multi-loop signal-cancelling antennas and RFID transponders. The system reduces interference from nearby metal structures in animal and livestock applications. The signal-cancelling antennas limit the spatial penetration of the antenna's magnetic field beyond the vicinity occupied by the RFID transponder. In Stewart's system, a single multi-loop antenna is used for transmit and receive functions. The loops of the signal-cancelling antenna are connected in series, all loops of the antenna have the same number of windings, and loops carry the same electrical current when driven by the signal.
In contrast to systems used for theft prevention and livestock inventory systems, some near-field magnetic communication systems use loop antennas to maximize the signal strength and distance rather than containing it to a small defined area. Additionally, some communications systems collocate the transmit and receive antennas in a closely spaced arrangement so that each communicating entity can both transmit and receive signals.